Special thanks to Markus Stamm for providing two samples of this lens!

Introduction

The Canon EF 300mm f/4 USM IS is part of Canon's professional grade L (Luxury) series.
While not exactly a cheap lens the 300mm f/4L IS is particularly interesting for many serious amateurs ("prosumers").
On APS-C DSLRs the field-of-view resembles a classic
480mm lens (full-format) suitable for wildlife or sports photography. The lens also accepts the Canon 1.4x and 2x
tele-converters so the range can be expanded quite a bit e.g. into bird photography. We also had a brief look
at the performance of this lens combined with the Canon EF 1.4x II resulting in a 420mm f/5.6 package (~670mm).
Using the 1.4x converter both AF as well as IS remains active on all EOS cameras (incl. film SLRs). Using
the 2x converter will deactivate AF on most models (due to a f/5.6 limit) except the EOS 3 and the EOS 1(D) series
(f/8 limit here). On most modern cameras EOS cameras IS will remain functional with the 2x converter.

The lens incorporates a first-generation Image Stabilizer (IS) with a claimed gain of 2 f-stops
for hand held photography. In the field I was able to confirm this statement. Due to the weight of the
lens it may even be a little more than that.
Unlike the consumer variants the IS system has 2 modes - the normal one for static shooting (horizontal + vertical
stabilization) and a panning mode (horizontal OR vertical stabilization - the panning direction is automatically
detected). Unlike the 2nd generation IS used in later designs the variant of the 300mm f/4 L IS has no tripod detection
so IS needs to be switched off here in order to avoid interference effects. On an unstable tripod or monopod the
IS can remain active - it just has to detect some kind of motion.
It is worth to mention that one of the two tested samples of this lens suffered from a very noisy (squeaky and raspy)
IS operation - despite some servicing by Canon.

The lens features a ring-type USM AF drive (rear focusing) with full-time manual focusing in one-shot AF mode.
AF speed feels extremely fast with this lens. The minimal focus distance is 1.5m resulting in a max. magnification
of ~1:4 (~1:3 with the 1.4x converter). Manual focusing feels very smooth as to be expected from an L grade lens.

The optical construction is made of 15 elements in 11 groups which is a rather unusual high number of elements
for a fix-focal lens. There're 2 UD (Ultra-low Dispersion) elements to correct aberrations.
The lens features 8 aperture blades. The filter size is 77mm.
At 1190g it is a rather heavy lens and with a dimension of 90x221mm it's also long - nothing for a colt bag
anymore. The lens has a build-in hood which can be extended simply by pushing it towards the front from its
base position. A good solution making a attachable hood obsolete and using polarizers isn't quite as awkward.